Disturbance in human leukocyte elastase (HLE) activity has been implicated as a major causative factor in the induction of pulmonary emphysema¹. Moreover, this serine protease has been suspected in other pathogeneses such as rheumatoid arthritis². As a result, HLE inhibitors are expected to be of considerable therapeutic interest.
Since the discovery of the very interesting inhibitory activity on HLE and other serine proteases of modified tert-butyl ester cephalosporins, new fields of investigations were focused on the synthesis and biological evaluation of structurally related analogues with a simplified chemical structure.
Different 5-aminothiazolium and 4-amino-4H-1,2,4-triazolium salts, 4-amino-1,2,4-triazoline-3-thiones and 4-amino-1,2,4- triazoline-3-ones, bearing in most cases a tert-butoxycarbonylmethyl moiety, have been previously selected as the free acid for their potential inhibitory activity on bacterial serine DD-peptidases³. They were now examined as potential enzyme inhibitors on a variety of serine proteases (HLE, porcin pancreatic elastase-PPE-, trypsin and chymotrypsin) and one aspartic acid protease (pepsin).
Because of the high reactivity of iminium-containing structures toward nucleophilic addition, the interaction of the thiazolium an triazolium salts with the nucleophilic hydroxyl function of the serine enzyme was expected to lead to the formation of a tetrahedral transition state analogue. Such an inhibitory mechanism may be compared to that of aldehydic and cetonic pseudopeptides or boronic acids on different serine proteases.
Interaction of the triazolinethiones and triazolinones with the serine enzyme might lead to the formation of a (thio) carbamoyl-enzyme instead of the acyl-enzyme. More stability to further hydrolysis of the new covalent complex to regenerate the free enzyme might be expected and irreversible inhibition might result. Carbamoyl-enzyme formation has been suggested to explain the inhibitory activity of physostigmine and neostigmine on serine cholinesterase.
Different thiazolium and triazolium salts have shown an inhibitory activity on trypsin and chymotrypsin at the millimolar level. Moreover, two triazolium salts and one triazolinone tert-butyl ester were found to be poor inhibitors of pepsin. No inhibition of HLE and PPE was observed with the different new compounds selected.
Before any definitive conclusion could be drawn, careful examination of structurally related compounds must be done in hope to select more powerful inhibitors of serine and aspartic proteases.

References :
1 Mittman C. (ed) Pulmonary Emphysema and Proteolysis, 1-537 (ACademic, New York, 1972)
2 Velvart M., Rheumatol. Int., 1, 121-130, 1981
3 Pirotte B., Dissert. Abstr. Int., 51, 1, 109-110, 1990.